1. Field of the Invention
This invention relates to a latex polymerization process for grafting styrene and acrylonitrile type monomers onto a rubber substrate in latex form. The process involves carrying out the graft polymerization reaction in the presence of latex seed particles.
2. Description of the Prior Art
Graft polymerization of monomers onto a preformed rubber substrate is known in the prior art as is evidenced by the teachings in U.S. Pat. Nos. 2,948,703, 3,373,227 and 3,742,092.
U.S. Pat. No. 2,948,703 teaches that the hot processability of ABS can be greatly improved by carrying out the graft polymerization in the presence of from 50 to 150 parts of seed per 100 parts of rubber.
U.S. Pat. No. 3,373,227 teaches the use of low molecular weight polystyrene particles in suspension polymerization in order to obtain improved flow properties in an ABS polymer.
U.S. Pat. No. 3,742,092 teaches the use of various seed particles (polystyrene, acrylonitrile/styrene, etc.) in the graft polymerization of methacrylonitrile and styrene onto a preformed rubber substrate for the purpose of improving the polymerization rates of methacrylonitrile.
It is conventional in the art to stabilize rubber latices with soaps. Some of these soaps such as tall oils, rosins and fatty acids, which have been saponified with ammonia, stabilize the rubber latex very effectively. However, these ammonium soaps inhibit the graft polymerization of styrene and acrylonitrile type monomers onto the rubber latex particles. As a result, the level of conversion of styrene and acrylonitrile type monomers to polymers is unacceptably low. In addition, the amount of polymer grafted onto the rubber substrate and the specific viscosity of the matrix polymer, i.e., the ungrafted polymer formed during the graft polymerization process, may also be unacceptable. This problem is further compounded when using conventional chain transfer agents such as terpinolene which also inhibits graft polymerization to some degree.
The use of mercaptan chain transfer agents which do not inhibit the polymerization, will give higher conversions of monomer to polymer. However, this gives rise to other problems in that the graft level of styrene and acrylonitrile type monomers onto the rubber substrate may be unacceptably high. With increasing graft levels, toughness of the polymeric product increases but its ability to be molded decreases (see FIG. I). The same holds true for an increase in the molecular weight of the matrix polymer (see FIG. II).
In acrylonitrile-butadiene-styrene graft polymers (ABS) the desired graft level for most commercial products is usually in the range of from 40 to 60 parts of SAN grafted onto 100 parts of polybutadiene rubber (pphr). Recently, considerably interest has been shown in replacing the polybutadiene rubber substrate with a polychloroprene rubber substrate in order to have a built-in halogen source in the polymer in order to obtain improved fire retardancy. The substitution of polychloroprene for the butadiene gives rise to new problems. The denser polychloroprene rubber, when grafted to a level of from 40 to 60 pphr, is increasingly resistant to flow. Thus, the graft levels which were desired in the polybutadiene based systems are unacceptably high for use in the polychloroprene based systems if good thermoplastic flow is to be obtained.
A need exists in the art for an improved graft polymerization process which can be used to graft styrene and acrylonitrile type monomers onto a polychloroprene rubber latex substrate which has been stabilized with a substance which normally inhibits polymerization of these type of monomers in order to provide grafted polymers with desired graft levels and matrix polymer with the desired specific viscosity.
In addition, a need exists in the art for a process which would provide grafted diene rubber compositions having good toughness and improved thermoplastic flow during melt processing and fabrication of these polymers.